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Clean up shm_mq cleanup.

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The logic around shm_mq_detach was a few bricks shy of a load, because
(contrary to the comments for shm_mq_attach) all it did was update the
shared shm_mq state.  That left us leaking a bit of process-local
memory, but much worse, the on_dsm_detach callback for shm_mq_detach
was still armed.  That means that whenever we ultimately detach from
the DSM segment, we'd run shm_mq_detach again for already-detached,
possibly long-dead queues.  This accidentally fails to fail today,
because we only ever re-use a shm_mq's memory for another shm_mq, and
multiple detach attempts on the last such shm_mq are fairly harmless.
But it's gonna bite us someday, so let's clean it up.

To do that, change shm_mq_detach's API so it takes a shm_mq_handle
not the underlying shm_mq.  This makes the callers simpler in most
cases anyway.  Also fix a few places in parallel.c that were just
pfree'ing the handle structs rather than doing proper cleanup.

Back-patch to v10 because of the risk that the revenant shm_mq_detach
callbacks would cause a live bug sometime.  Since this is an API
change, it's too late to do it in 9.6.  (We could make a variant
patch that preserves API, but I'm not excited enough to do that.)

Discussion: https://postgr.es/m/8670.1504192177@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2017-08-31 15:10:24 -04:00
parent 4b1dd62a25
commit 6708e447ef
5 changed files with 51 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
src/backend/libpq/pqmq.c
View File

@@ -21,7 +21,6 @@
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
static shm_mq *pq_mq;
static shm_mq_handle *pq_mq_handle;
static bool pq_mq_busy = false;
static pid_t pq_mq_parallel_master_pid = 0;
@@ -56,7 +55,6 @@ void
pq_redirect_to_shm_mq(dsm_segment *seg, shm_mq_handle *mqh)
{
PqCommMethods = &PqCommMqMethods;
pq_mq = shm_mq_get_queue(mqh);
pq_mq_handle = mqh;
whereToSendOutput = DestRemote;
FrontendProtocol = PG_PROTOCOL_LATEST;
@@ -70,7 +68,6 @@ pq_redirect_to_shm_mq(dsm_segment *seg, shm_mq_handle *mqh)
static void
pq_cleanup_redirect_to_shm_mq(dsm_segment *seg, Datum arg)
{
pq_mq = NULL;
pq_mq_handle = NULL;
whereToSendOutput = DestNone;
}
@@ -135,9 +132,8 @@ mq_putmessage(char msgtype, const char *s, size_t len)
*/
if (pq_mq_busy)
{
if (pq_mq != NULL)
shm_mq_detach(pq_mq);
pq_mq = NULL;
if (pq_mq_handle != NULL)
shm_mq_detach(pq_mq_handle);
pq_mq_handle = NULL;
return EOF;
}
@@ -148,7 +144,7 @@ mq_putmessage(char msgtype, const char *s, size_t len)
* be generated late in the shutdown sequence, after all DSMs have already
* been detached.
*/
if (pq_mq == NULL)
if (pq_mq_handle == NULL)
return 0;
pq_mq_busy = true;